Fertilizers are composed primarily of three macro-nutrients with other micro-nutrients and minerals. The fertilizer numbers relate to the percentage of macro-nutrients, by weight, in the bag. The three numbers designate nitrogen (N), phosphorus (P), and potassium (K) content respectively. A mixture labeled 20-15-10 would contain 20% nitrogen, 15% phosphorus, and 10% potassium, in that order.
The letters N-P-K are the chemical designations for the elements nitrogen, phosphorus, and potassium. Fertilizer numbers are provided by the manufacturer to assist in the correct fertilizer selection. The N-P-K designation is required by law and the contents are guaranteed to be the correct ratio of nutrients. If the ratio contains a fourth number, the manufacturer has included another nutrient such as sulfur, iron, or zinc.
When the fertilizer numbers are added together, they do not add up to 100%. The remainder of the weight is in the form of fillers which may contain minor nutrients and minerals. The purpose of filler is to allow a more uniform application of the nutrients without excessive overlapping. Excessive concentration of nutrients could harm the plants.
A 100 pound (45 kilogram) bag of fertilizer labeled 5-5-5 would have 5 pounds (2.27 kilograms) each of nitrogen, phosphorus, and potassium with the remaining 85 pounds (38.55 kilograms) being filler. A 5-5-5 fertilizer will typically provide the nutrients nearly all plants need for healthy growth.
The fertilizer numbers are provided to ensure that you apply the right kind of fertilizer for your soil conditions. The kind of plants being grown and the soil requirements will determine the N-P-K analysis selection. A soil test will reveal the nutrients needed for healthy plant growth. Soil test kits are available at local home or garden supply stores and from a number of online retailers.
Organic fertilizer numbers need to be interpreted differently. Organic fertilizers release nutrients at a slower rate. The N-P-K ratio must express nutrients that are immediately available. The ratio for organic fertilizers is typically lower than synthetic fertilizer. Fertilizers that contain both organic and synthetic ingredients must be labeled.
Complete fertilizers contain a percentage of all three nutrients such as 5-10-20. Incomplete or specialized fertilizers lack one or more macro-nutrients such as 0-20-20, which lacks nitrogen. Additional ingredients are usually listed on the side label and may include micro-nutrients, minerals, and organic matter. Fertilizers with higher numbers are typically more expensive because they contain more nutrients.